Conventional exploratory and well drilling operations use the same basic approach for sinking well casing down the drilled hole. Sections of casing are fitted together at ground level and driven from the top of the hole downwardly as drilling progresses. In essence, the drill string is progressively pushed down the drill hole.
Frictional resistance to downward movement increases as the hole depth increases, therefore correspondingly increasing the requirements for case driving forces. Long drill strings have a tendency to buckle as a long column under compression. The surrounding earth prevents such buckling at the cost of increased friction against the sides of the drilled hole. More driving force is therefore required as the length of casing increases. A casing driven from the top of the hole will normally follow the drilled hole but not with the desired degree of accuracy, especially in soft ground. Section welds can easily become damaged due to constant lateral shifting (partial buckling) under the high compressive forces incurred.
The above problems were recognized to a limited degree by Davey, Sr. et al in U.S. Pat. No. 3,190,378 granted June 22, 1965. The Davey casing driving mechanism makes use of apparatus for both drilling and for pulling casing downwardly into a drilled hole. A rotary drill bit is releasably connected to a casing shoe mounted to the bottom of the casing. As the rotary drill bit rotates, inverted L shaped brackets on the rotary bit engage dogs that project inwardly from the casing shoe. The casing shoe is mounted to the bottom of the casing and rotates with the rotary drill bit. The bit rotates the casing shoe and pulls the casing downwardly as the drilling progresses. At the end of the drilling operation, the drill tool is rotated in an opposite direction to disengage the rotary bit and the inverted L shaped brackets from the dogs, thus enabling retraction of the rotary bit and the drill string up through the casing.
The Davey cutting and casing driving shoe is extremely expensive. The shoe must have especially hardened and formed drill teeth at lower ends and an appropriate sealed bearing at upward ends where the shoe is connected to the casing bottom. The bearing must be constructed both to withstand downward forces imparted by the drill tool and to allow relatively free rotation of the shoe so that torsional forces are not transmitted from the rotating drill bit to the casing. Additionally, should the bearing fail or freeze, the shoe will transmit torsional forces directly to the casing as the drill bit rotates. Such a failure could result in damage to the casing and would require the entire string to be removed from the drilled hole for repair.
A pile driving device is shown in the U.S. Patent to Blumenthal, U.S. Pat. No. 1,908,217 granted May 9, 1933. Blumenthal discloses a drive point that is hammered into the ground by a downhole pile driver. The pile shell is pulled downwardly by the downhole pile driver. The Blumenthal device is used exclusively for driving pilings and does not suggest use in a drilling operation in which earth material must be removed from the hole. Blumenthal, however, exemplifies the desirability for downhole "driving" of a piling shell to prevent compressive damage of the piling shell and to decrease the force required to move the piling shell down the hole.
Blumenthal makes use of a transverse bar that is affixed to inward surfaces of the casing as an anvil surface. The pile driving device strikes a top surface of the rod to transmit downward driving forces to the attached piling. The area of contact between the bar and piling is limited to the cross-sectional area of the bar where it is attached to the piling. Thus, tremendous impact forces are to be absorbed across a relatively small cross-sectional area of the rod. Furthermore, the bar extends completely across the piling interior, blocking passage of the impact device to areas below the rod.
The above described apparatus disclosed by Blumenthal and Davey clearly illustrate the desirability to provide some form of downhole driving for casing or pilings. However, both are plagued with limitations, especially in the area of the driving "anvil" surface that is provided on the casing to transmit forces from the driving member to the casing. Davey, for example, uses inwardly projecting dogs on the rotatable drill shoe. Since the dogs rotate relatively freely within the casing, there is no fixed position about the casing axis specifically provided for imparting downward driving force to the casing. The rotating dogs, instead, transmit downward driving force continuously during rotation. The result is combined downward force and a resultant torsional force due to rotation, even though the rotational forces are minimized (hopefully) by the bearing mounts.
Davey's apparatus is used strictly for rotary drills. It would not operate effectively, if at all, in conjunction with present percussion drilling equipment. The drilling mechanisms is percussion drilling move in vertical, up and down hammering strokes. Therefore, driving dogs such as those disclosed by Davey, mounted on a rotating shoe, could not be trusted to remain in the same angular position about the axis of the casing for proper alignment with hammering surfaces on the impact drill tool. Furthermore, the bearings mounting the shoe to the casing bottom would more than likely fail under the continuous impact driving forces. Blumenthal, on the other hand, provides a stationary driving surface. However, such surface is mounted in such a way that would not permit its use by impact drilling tools, since the driving surface extends entirely across the casing. Furthermore, the points of attachment of the driving surfaces could easily fail if adapted to fit within a standard well casing due to the small cross-sectional areas of engagement between the striking surfaces and casing walls.
The present case driving anvils mount through apertures formed in conventional well casings upward from the casing bottom and are fixed in relation to the casing. The anvil surface remains in position in alignment with the impact driving hammer. The anvil and casing are relatively stationary so there are not moving parts to malfunction or break. Furthermore, the present anvil structure is provided to reinforce the casing area lost through the mounting apertures, providing a large area of contact with the casing and attachment to the casing at points spaced from the driving surface and aperture so forces are more evenly distributed to the casing during impact.